APPLICATION OF GEOELECTRICAL RESISTIVITY IMAGING TO INVESTIGATE GROUNDWATER POTENTIAL

ABSTRACT

 Geoelectrical resistivity imaging using both 2D and Vertical Electrical Sounding (1D) method was carried out in Atan , Ado/Odo Ota Local Government, Ogun State ,Nigeria using the PAS earth resistivity meter. Three profiles was carried out using both Wenner and Schlumberger array configuration. The data was interpreted using RES2DINV for 2D and computer iteration method (WinResist) for VES. Results showed that there are five major layers which described the geological structure of the study area. The resistivity values ranges between 1200Ωm to 4000Ωm, 500Ωm to 1000Ωm, 400Ωm to 800Ωm , 200Ωm to 400Ωm and 60.7 Ωm to approximately 100Ωm respectively according to the structures. VES1, VES 2, VES 3 and VES 4 apparent resistivity curve shows the different layers with their corresponding thicknesses. The study area revealed that the depth to the aquifer ranges between 130m t0 140m.

CHAPTER ONE

  INTRODUCTION

 Groundwater has an excellent microbiological quality and generally adequate chemical quality for most uses. Nine major chemical constituents (Na, Ca, Mg, KHCO3, Cl, SO4, NO3 and Si) make up 99% of the solute content of natural groundwater. The proportion of these constituents reflects the geology and history of the groundwater. Minor and trace constituents make up the remaining 1% of the total, and their presence (or absence) can occasionally give rise to health problems or make them unacceptable for human or animal use (British Geological Survey to Africa) .

Groundwater and other mineral resources such as hydrocarbons and solid minerals are of great abundance in Nigeria. The true riches of any country depend on its ability to provide for its dwellers. Potable water is one of the major resources that a citizen of any nation can benefit from (Alile, 2008). This is so because water is a free course of nature. It is a gift of God to mankind. This wonderful resources have transcends so many generation because of its value to human life. Water is one of the major determinants of economical development. Water has found its usefulness in every human endeavour such as manufacturing industry, agricultural industry, transportation industry, construction industry, home usage and so on. Because of the importance of this resources new technology have been developed in search of this resources. Water search has extended from surface to ground exploration. There are parameters that characterized groundwater such as conductivity, porosity, permeability and transmissivity. All these parameters are determined using any geophysical methods such as magnetic methods, gravity methods etc. In this research work, electrical resistivity method was employed for groundwater exploration (Alile, 2008). Geoelectrical resistivity surveys are often used to search for groundwater in both porous and fissured media. Clean sands and gravels which have porosities always make good aquifers when saturated with fresh water which can easily be differentiated from lower- resistivity impermeable clays and marls and also from bedrock which is mainly of much higher resistivity (Sharma, 1997). Areas where the groundwater is significantly saline, the aquifers resistivity is reduced greatly and resistivity surveying can delineate the boundaries of the body of saline water (Sharma, 1997).

Geoelectrical resistivity method has developed greatly and has become an important instrument in hydrological studies, mineral prospecting and mining as well as in environmental and engineering applications. (Griffiths et al; 1990; Alile et al; 2010; Griffiths and Barker, 1993; Dahlin and Loke; Aizebeokhai et al; 2010). This underlying principle of measuring subsurface variation using electrical resistivity within the earth was developed by Schlumberger who conducted the first experiment in 1912 in the field of Normandy and the same idea was also developed by frank Wenner in the united State of America. (Kunetz, 1966). This geoelectrical resistivity method has been found useful in locating groundwater in fissured rock, mapping of plumes, mapping of boundaries of saline groundwater and exploration of geothermal fluids.

Due to the successful application of geoelectrical resistivity over the years in groundwater exploration, this propels me to adopt geoelectrical resistivity method to investigate groundwater potential and geological structure of the study area However; geoelectrical resistivity surveys have undergone significant changes in the last three decades. The traditional horizontal layering technique for investigating geoelectical resistivity data are rapidly being replaced with 2- dimensional and 3- dimensional models of interpretation especially in complex and heterogeneous subsurface media.

Field techniques have advanced from manual measurements made at separate and independent points to the use of automated machine called terrameter with multi-electrode array along the measurement profile. Till 1980s, data acquisition was more or less carried out manually and this is demanding and slow and the quality of the measured data is poor. Therefore a range of fast automated multi-electrode and multichannel data acquisition system now exists that follows flexibility in the acquisition of geoelectrical resistivity data. (Barker, 1981; Stummer and Maurer, 2001; Auken et al; 2006).

1.2 GEOLOGICAL DESCRIPTION OF UNDERGROUND WATER

Groundwater can be found almost everywhere. The water table may be deep or shallow and may rise or fall depending on many factors.

Heavy rains or snow may cause the water table to rise or fall. Groundwater is stored in and moves slowly through layers of soil, sand and rocks called aquifers. The speed of groundwater flows depends on the size of the spaces in the soil or rock and how well the space is connected. Groundwater is brought to the surface naturally through a spring or can be discharged into lakes and streams. This water can also be extracted through a well drilled into the aquifers. A well is a pipe in the ground that is filled with water. This water can then be brought to the surface by a pump. Shallow wells may go dry if the water table falls below the bottom of the well. Some wells called artesian well do not need a pump because of natural pressures that force the water up and out of the well.

Groundwater supplies are replenished or recharged by rain and snow melt. In some areas of the world, people face serious water shortages because groundwater is used faster than it is naturally replenished. In other area groundwater is polluted by human activities. Groundwater is a natural resource that is use for drinking, recreation, industry and agriculture. In areas where material above the aquifer is permeable pollutants can sink into the groundwater. Groundwater can be polluted by landfills, septic tanks, leakages of underground gas tanks and from overuse of fertilizers and pesticides.

1.2.1 THE OCCURRENCE OF GROUNDWATER.

 The groundwater is a term used for water which occurs beneath the ground surface. It is an important constituent of hydrological cycle and plays a major role in augmenting water supply to meet the major increasing demands in various sectors. Groundwater occurs in the upper layers of the earth’s crust. These layers consist of igneous, sedimentary and metamorphic rocks. During their origin and later evolution, these rocks develop porous and permeable structures containing pore spaces. Within these pores, the water of meteoric, juvenile, connate or metamorphic origin occurs in both liquid and gaseous phases along with other gases and liquid such as hydrocarbons and magma. Meteoric water includes rain water, lake and river waters. Juvenile water is assumed to be derived from the mantle during degassing processes (Bredchoeft and Norton, 1990; Rai, 2004). Formation water is the water trapped during the deposition of sediments and produced during diagenetic reaction. Metamorphic waters are derived from the dehydration of hydroxyl bearing minerals through rising pressure and temperature. These fluids are subjected to a wide variety of stresses such as recharge due to precipitation, return flow from irrigation, seapage from canals, lakes, ponds etc. As a result, a groundwater regime consisting of geological formation and groundwater is established.

1.2.2 THE MOVEMENT OF GROUNDWATER

 Groundwater, apart from being a major source of water supply, groundwater is the most important geological agent among all fluids of the earth’s system and plays important role in many geological processes. This is so because of its moving ability and its ability to interact with the surrounding environments. The interaction of groundwater with its surrounding generates various natural process, products and conditions and the moving ability helps in self organizing the effects of interaction within the flow system. Three main types of interactions have been identified; they are chemical, mechanical and kinetic. Accordingly, the processes of these interactions are classified as chemical processes, mechanical processes and kinetic processes. (Toth, 1999; Rai, 2002). Chemical processes include dissolution, hydration, hydrolysis, oxidation, reduction, chemical precipitation and base exchange. Pore – pressure change and lubrication are the two most important physical processes that affect many geological phenomena. Reduction or increases in pore pressures affect the magnitude and direction of groundwater velocity which ultimately affect the type, rate and direction of chemical reactions, the solubility of minerals etc. They also affect the strength and integrity of rocks, leading to their deformation. Lubrication by water of discontinuity boundaries in the rock frame work such as grain surfaces in soils and in unconsolidated sediments or fracture and fault planes in hard rock reduces friction and enhances the effect of shear stresses. As a consequence, shear movements of soil and rocks can be induced along the discontinuities which may lead to the occurrence of land subsidence, landslides and earthquake like geologic phenomena.

A wide variety of matter in many different forms such as aqueous solutions of organic and inorganic ions, matter in colloidal forms or larger-sized suspended grains, gases, molecules of liquid hydrocarbons, viruses and bacteria are transported by groundwater movement. The importance of transport of these matter resulted in the leaching and removing of minerals from soils and rocks, carrying nutrients to surface water bodies, building and emigrating deposits of metallic and non-metallic minerals and hydrocarbons, causing washing and biodegradation of ore deposits and hydrocarbon accumulation and concentrating contaminants at suitable subsurface locations. Heat transport by moving groundwater leads to the formation of hot springs, hydrothermal ore deposits or other types of geothermal anomalies.

1.2.3 TRANSMISSION OF GROUNDWATER.

The dynamics of groundwater is mostly influenced by four hydrological processes. These are infiltration, recharge, leakage and withdrawal. These are processes through which groundwater is transmitted. Infiltration is the process of entry of surface water into the soil through ground surface. The rate of infiltration is defined as the volume of water entering into the ground surface through its unit cross-sectional area in unit time. The infiltration rate is known to vary with time which is initially decreases due to dispersion and swelling of soil particles after wetting (Alile, 2008). Then for a shorter period it increases due to the release of entrapped air from the pores. Thereafter, it decreases more or less in exponential form due to the clogging of the soil pores at the bottom of the surface reservoir. When the infiltration rate drops below a prescribed lower limit such that it is no more economical for aquifer recharging, then the infiltration operation is disconnected for some time. After drying, cleaning and if necessary scraping of the silty bottom of the recharge basin, infiltration is brought back to its almost initial value and the basin is put back to use for the next phase of the infiltration operation ( Bear, 1979; Husiman and Olsthoorn, 1983; Zomonody, 1991; Deta,

1995; Dickenson and Bachman, 1995; Mousavi and Rezai,1999). Several models and Horton model have been proposed to describe the infiltration rate(singh, 1989) . Among them the Horton model is widely used which is given in the following form.

Where I(t) is the infiltration rate at time t. Io and Ic are the initial and final values of the infiltration rate. Β is a proportionality factor dependent on soil type and initial moisture content. However, this model describes only exponential decay of the infiltration rate for one cycle of infiltration. Such a composite nature of decreasing and increasing filtration rates for any number of dimensions can be in a better way approximated by using linear elements of different lengths and slopes depending on the nature of infiltration rates. The mathematical expression of time varying infiltration rate approximated by this scheme is given by:

where rij and cij are the slope and intercept of the jth line element of the infiltration rate for the ith basin and K is the number of elements. Infiltration rate is used for modeling groundwater flow in unsaturated zone. Diaw et al;(2003), Manglik et al; (1997), have used this scheme of approximation of time varying infiltration rate for simulation of groundwater flow in variably saturated aquifer system.Fig 1.1: Approximation of time varying infiltration rate

1.2.4 RECHARGE

The process of entering of infiltrated water into the saturated zone. The rate of recharge is defined as the volume of water which enters into the saturated zone through per unit cross sectional area and per unit time. This rate of recharge largely depends on the infiltration rate and follow pattern of variation more or less similar to that of infiltration rate with some time lag and less intensity. Just like infiltration rate is approximated the recharge rate can be approximated also by using linear element of different lengths and slopes depending on the nature of variation of recharge rates.

1.2.5 LEAKAGE

The leakage takes place between two adjacent aquifers through apportion of semi permeable boundary due to difference in hydraulic pressure. The leakage rate is defined as the volume of water leaving or entering the aquifers through a unit cross sectional area of semi permeable boundary in unit time. Leakage from an overlying or underlying aquifer with piezometric head, Ф_c into aquifer with piezometric head Ф is given by: Where k and b are the hydraulic conductivity and thickness respectively of the semi permeable layer.

1.2.6 WITHDRAWAL

 The withdrawal of groundwater is carried out by pumping. It is also takes place naturally at the interface of aquifer and surface water body having water level lower than the level of saturation. The pumping rate is defined as the volume of water withdrawal from a unit cross sectional area of the aquifer in unit time. Both leakage and withdrawal rates can also be approximated by the schemes expressed by equation (2). In this case shapes of leakage sites and the wells should be in the rectangular form.

1.2.7  AQUIFERS

 Aquifers are the geological formations which can store water as well as allow the flow of significant amount of water through their pores under ordinary field conditions. In these formations pores are interconnected to allow the flow of groundwater. Examples are sand, sandstone, weathered rocks, fractured rocks etc. If the aquifer is bounded by two impermeable formations from top and bottom is called a confined aquifer. Water level in a piezometer penetrating a confined aquifer is referred to as the piezometeric surface. The pressure at the piezometric surface is equal to the atmospheric pressure. Water level in a well penetrating a confined aquifer represent piezometric surface. If the piezometric surface is above the ground surface, then a well located in this region will discharge groundwater on the ground surface without pumping. This segment of confined aquifer is known as artesian aquifer and the well is known as artesian well. If the upper boundary of the aquifer is the water table or phreatic surface is called an unconfined aquifer. The pressure at the water table is equal to atmospheric pressure. The pressure above the water table is less than the atmospheric pressure whereas below the water table it is more than the atmospheric pressure.

An unconfined aquifer or part of it that rest on a semi pervious layer is a leaky unconfined aquifer. Confined aquifer that has at least one semi pervious formation is called a leaky confined aquifer. Perched aquifers are special cases of unconfined aquifers in which a small impervious layer supports a groundwater body above the main water table. Different types of aquifers are shown in figure 3. The same layer of an aquifer can represent different form of aquifers depending on the prevailing physical conditions. As shown in the figure 3, the piezometric surface within the a-b segment of the lower confined aquifers falls below the top of the aquifer. In this segment, piezometric surface and water table coincides and the aquifers become unconfined. The c-d segment represents as an artesian aquifer and the e-f segment as a leaky confined aquifer. Other than being the source of fresh water, aquifers are used for other purposes of groundwater resource management such as storage reservoir, as a filter for water of inferior quality injected into the aquifers etc. The geological formation that allows the flow of water at very low rate compare to the aquifer is called Aquitard. The formation that contains water but is incapable of transmitting significant quantities of groundwater under ordinary field conditions is referred to as Aquiclude. Clay is an example of aquiclude. The geological formation that neither store water nor allow the flow of water through them is called Aquifuge. Hard rock formations such as granites and basalts which are free from fractures,faults or weathering are example of Aquifuge

1.3 Location of the Study Area.

 The location of the study area is Atan in Ado- Odo/Ota Local Government Area of Ogun State Southwest Nigeria. The Local Government lies at longititude 006^o.40.228 and latitude 003^o05.397 .

Fig 1.5:  Geological Map of Ogun State showing the study Area

1.4 Aim and Objectives

The aim of this research work is to determine the groundwater potential in Atan.

The objectives of the study are:

• To determine the availability of groundwater resources in Atan using geoelectrical resistivity method.
• To determine the depth of groundwater of the study area using vertical electrical sounding.
• To determine the geological structure of the study area.

1.5 Scope of the Study

The scopes of the study are:

• To analyze the structure of underground characteristics.
• To analyze the resistivity data of the study area by engaging the 2-D Resinverse software.

1.6 Justification of the study.

 More geophysical imaging research or study of the subsurface earth need to be done adopting the use of geoelectrical resistivity method. This is because geoelectrical resistivity method has proven relevant and the reliability of the method is not in doubt. The method can probe deep into the earth and therefore has the ability to detect the depth to groundwater layer and to determine the geological structure of the study area.

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